1. Field of the Invention
The present invention generally relates to an automatic control system for controlling a chemical reaction in order to maximize efficiency, and more particularly to an automatic control system for controlling the operation of the attack tank in a phosphoric acid plant in order to minimize the cost of production of phosphoric acid. Phosphoric acid (H.sub.3 PO.sub.4) is an important intermediate chemical product. It is used primarily by the fertilizer industry, but is also used in a number of other areas such as in detergents, water treatment and food products.
Phosphoric acid is primarily produced by what is known in the art as the "wet process." Using the wet process, phosphate rock, which includes calcium, phosphate and a number of impurities, is mined, beneficiated (concentrated) and then ground dry or wet through the use of ball mills or rod mills. The ground rock is fed into an attack tank or reactor and reacted (digested) with concentrated sulfuric acid (H.sub.2 SO.sub.4). This process may be represented by: EQU Ca.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 +6H.sub.2 O=2H.sub.3 PO.sub.4 +3CaSO.sub.4 .times.2H.sub.2 O (eqn. 1).
This reaction produces phosphoric acid (H.sub.3 PO.sub.4,) and calcium sulfate (CaSO.sub.4) which is a waste product and is commonly referred to as gypsum. This process is illustrated in FIG. 1. Phosphorous pentoxide (P.sub.2 O.sub.5) is the term most often used to measure plant production rather than H.sub.3 PO.sub.4. Plant output is referred to in tons of P.sub.2 O.sub.5, not actual tons of acid. Approximately 4 tons of 30% phosphoric acid equals 1 ton of P.sub.2 O.sub.5 (Phosphoric acid and P.sub.2 O.sub.5 may be used interchangeably when referring to plant output).
The goal of a phosphoric acid plant is to maximize the production of phosphoric acid from the limited amount of phosphate rock, and thus, minimize any potential losses of phosphoric acid. An important factor influencing the efficiency of a phosphoric acid plant is the amount of free or excess sulfate (SO.sub.4)(measured as weight % of excess sulfate) in the reactor or attack tank. The amount of excess sulfate constitutes the amount of sulfate not reacted with calcium or in the form of CaSO.sub.4. On this basis, "excess sulfate" may be either a positive or a negative number (positive representing an excess of sulfate, and negative representing a deficiency of sulfate).
The process shown in equation 1 produces two chemical products: calcium sulfate crystals (gypsum--a waste product), and phosphoric acid. A properly operating phosphoric acid plant results in a high yield of P.sub.2 O.sub.5 recovery from the attack tank slurry, and the formation of calcium sulfate crystals which contain little unreacted phosphate, and have a shape, size and porosity that filter easily. Level of excess sulfate is one of the most important factors governing crystallization quality.
There are primarily 3 types of P.sub.2 O.sub.5 losses that may occur in a phosphoric acid process. First are lattice losses or co-crystallized losses which occur due to the precipitation or co-crystallization of P.sub.2 O.sub.5 with the calcium sulfate. Lattice losses tend to increase as the amount of excess sulfate in the reactor is decreased. Second are coated rock losses which are due to an over-vigorous calcium sulfate (gypsum) crystallization which in turn causes phosphate rock particles to be covered by a gypsum shroud before the particle can completely react, thus shielding the particles from further attack (reaction) by the sulfuric acid. Any phosphate rock particles which become coated with gypsum result in a loss of P.sub.2 O.sub.5 production because this unreacted phosphate is thrown out along with the other solid matter (gypsum) in the tank slurry after filtering. Coated rock losses increase as the excess sulfate in the attack tank increases. Coated rock losses may also vary to some extent with the quality and with the fineness of the phosphate rock particles (rock quality is usually measured as a grade (a numerical value) in units of bone phosphate of lime (BPL)). Third are acid losses which occur due to inadequate filtration. Gypsum crystal size, and thus filtration characteristics, are primarily determined by the variability in the excess sulfate. In a properly operated phosphoric acid plant lattice losses (1) and coated rock losses (2) usually account for a majority of the acid losses encountered.
FIG. 2 illustrates theoretically how the amount of P.sub.2 O.sub.5 recovery is heavily dependent on the amount of excess sulfate in the reactor. The graph of FIG. 2 represents how the recovery of phosphoric acid (P.sub.2 O.sub.5) varies as the amount of excess sulfate (wt. % excess sulfate) changes between 0% and 4%. For this theoretical plant, P.sub.2 O.sub.5 recovery is maximized when the excess sulfate in the reactor is maintained at approximately 2%. For a different plant or if the plant operation rate is increased or decreased, the optimum operating point (target) may be different. Periodic efficiency tests can reveal any changes in this optimum operating point. If the excess sulfate in the reactor decreases below the optimum point of about 2% excess sulfate, increased lattice losses cause a decrease in P.sub.2 O.sub.5 recovery of the plant. Similarly, if the plant is operated above the optimum point of 2% excess sulfate, P.sub.2 O.sub.5 recovery also suffers due to increased coated rock losses. Therefore, to maximize production of P.sub.2 O.sub.5, the amount of excess sulfate in the reactor should be maintained as close as possible to this predetermined optimum (target) operating point. The determination of the optimum point of operation for a particular facility is well within the skill of the phosphoric acid process control art.
Many problems may arise when attempting to maintain the excess sulfate in the reactor at this predetermined optimum point. If, for example, the optimum operating point was found to be 0% excess sulfate, then the amount of sulfate desired in the reactor is that amount that allows a complete reaction of all phosphate and results in no (0%) excess sulfate present. The amount or flow rate of sulfuric acid necessary to achieve this optimum operating point will need to vary based on several factors including (i) the mass flow rate of the rock (usually measured in tons/hour), (ii) the quality or grade of the rock CaO content), (iii) rock fineness, (iv) reactor temperature, etc. Therefore, the flow rate of sulfuric acid which is fed into the reactor (attack tank) necessary to achieve the optimum operating point (sulfuric acid demand) will be different for different mass flow rates, rock qualities and fineness of incoming rock.
The quality or grade of the rock is determined by the percent of calcium present in the rock and the type and amount of impurities also in the rock. The fineness of the rock is determined by the rock itself and the process and equipment used to grind the rock. The mass flow rate of the rock, which may be considered to be the operating rate or speed of the plant, can be measured using a magnetic flow meter (if rock is fed as slurry) to determine volume flow of rock per unit time, and a nuclear density meter to measure the density of the rock slurry. Using these two values, the mass flow rate of the rock (tons/hour) may be calculated. For any particular rock type (including fineness, quality, etc.) there exists some demand of sulfuric acid (S) per ton of rock (R), which may be represented as the ratio S/R, which produces the desired level of excess sulfate in the reactor. This may be referred to as the set ratio. The excess sulfate level in the reactor may be tested manually using well known laboratory methods, such as by precipitation as barium sulfate and by measuring turbidity. The acid flow rate and the rock mass flow rate must be monitored and adjusted to maintain the excess sulfate level in the reactor equal to this predetermined level. For example, in one approach the rock mass flow rate may be held approximately constant and the flow rate of the sulfuric acid then adjusted to maintain the desired S/R ratio which produces the desired excess sulfate level.
However, several additional problems exist which often hinder attempts to run the plant at this optimum operating point. First, the quality and fineness of the incoming rock are constantly changing because the quality of the rock and type of impurities in the rock vary widely among different ore sites, and even at the same site. Thus, the desired S/R ratio must frequently be updated in order to maintain the reactor at the optimum level of excess sulfate. Unexpected difficulties such as mechanical problems (i.e., broken pumps, stuck valves, and the like) often prevent maintaining the flow rates at a known constant level, and may cause the excess sulfate level in the reactor to change drastically before any malfunction is detected. Even if the sulfuric acid flow rate is properly adjusted for the particular incoming rock, the excess sulfate already present in the reactor may not be correct. Therefore, an additional adjustment in the sulfuric acid flow rate must be made to correct for any sulfate excess or deficiency already in the reactor. In addition, after an adjustment in the sulfuric acid flow rate has been made, there is an unavoidable lag time before the excess sulfate level in the reactor changes in response to the new acid flow rate.
2. Description Of Related Art
Prior systems have attempted to control the production of phosphoric acid and the amount of excess sulfate in the reactor. Historically, sulfate control has been achieved by using large tanks for the attack process with periodic manual measurement of the excess sulfate in the tank, followed by adjusting, as needed, the acid or rock feed rates. Because an operator obtained little guidance about the myriad of factors influencing operation in such a system and thus was left in the dark as to what changes were required, different operators often would make entirely different changes under the exact same operating conditions. Using this procedure, it has only been possible to maintain an excess sulfate value that typically has a standard deviation of about 0.5 around the desired optimum.
There have been attempts to improve sulfate control. Generally, the approaches utilize continuous analysis of the rock (for calcium or P.sub.2 O.sub.5 content) or continuous analysis of the sulfate in the reactor or attack tank. However, these measurement devices were prone to breakage (clogged pipes, corrosion of internal parts, etc.) and thus usually required full time maintenance. Because of prohibitively high costs (initial cost and maintenance costs) and unstable measurements, such on-line measurement techniques have been abandoned by most plants. Even with constant measurement of the sulfate level in the attack tank, maintaining the sulfate level at the optimum point is still extremely difficult because generally one does not know what types of changes are necessary as well as what degree of change is needed to return the sulfate level to the desired optimum.
In 1988, IMC Fertilizer, Inc., New Wales, Fla., implemented an "advisory system" using a computer to assist in plant control. The computer for this system required the operator to input data hourly into the computer on the measured level of excess sulfate in the attack tank and or other operating conditions of the reactor. The computer then performed a mass balance to determine if a rock change had occurred in the last hour. Next, the computer compared the expected level on the last correction made of excess sulfate vs. the actual level of excess sulfate for the last 4 hours to determine if the calculations needed to be adjusted for any changes in the reactor, etc. If the measured excess sulfate level was within a certain narrow control range of the target (optimum), a change in sulfuric acid flow was recommended to the operator. If the measured level of excess sulfate was outside the narrow control range, then both a sulfuric acid spike or pulse (positive or negative) and a change in the sulfuric acid flow rate were recommended to the operator. The data and calculations were saved by the computer for further processing.
Because changes in the acid flow rate only compensated for any change in rock mass flow rate, rock quality, etc. in some cases, the recommended sulfuric acid spike was necessary to correct the sulfate level of the material already in the tank. Even though this system constituted an improvement over prior art systems and in practice reduced the standard deviation of the level of excess sulfate in the attack tank around the desired sulfate level by approximately 60%, many problems existed with this system which prevented the operation of the plant continuously at the optimum or target sulfate level.
First, even though the computer recommended a sulfuric acid flow rate of, for example 400 gallons/min., the operator never knew what the actual flow rate was because of unavoidable frequent fluctuations in the flow rate. Thus, the actual flow rate over the last time period actually may have averaged 395 gallons/min. even though the acid flow rate set point was 400 gal/min. This may be due to a pump which did not consistently supply the selected flow rate of acid. Under such circumstances, the total amount of acid added over a particular sampling period had to be estimated using the acid set point because the actual acid feed rate was not known. (No integration was performed.) This severely limited performance. Second, even though the computer may have recommended use of an acid spike of, for example, an additional 200 gallons/min. for 10 minutes to adjust for the material already in the tank, and followed by a readjustment to the original acid flow rate, the actual spike created may be something quite different from what was recommended due to mechanical limitations such as the valves not opening/closing immediately or pumps not capable of providing the increased flow rate, imperfect pump performance, etc. Third, the data processing used to calculate the desired acid flow rate, acid spikes etc., was relatively simple and provided less than superior plant performance. In addition, the data used to calculate recommendations, including past flow rates and past changes made, was never actually known for reasons as discussed above. Therefore, there remained a need for a control system which accurately controlled the various components and procedures in a phosphoric acid plant, including the accurate control of the excess sulfate level in the reactor or attack tank.